The membranes of all living cells contain specific transport proteins that enable the cell to accumulate nutrients and to maintain appropriate concentration gradients of ions. We will study the structure and function of a specific active transport system in order to gain some understanding of how this process occurs. The transport system that will be studied is the maltose/maltodextrin transport system of Escherichia coli. This system is composed of five protein subunits which enable the cell to actively pump maltose and longer polymers of glucose (maltodextrins) into the cytoplasm. We will use biochemical techniques to localize the sites that interact with substrate molecules. In addition we will use specific labels for locating the site(s) at which the energy donor interacts with the system. Finally we will define the location at which the water soluble maltose binding protein interacts with the membrane bound components of the system. We will also use a genetic approach to complement these biochemical experiments. We will isolate mutants with altered substrate specificity to localize the substrate recognition site genetically. In addition we will isolate mutants in which the interaction of the membrane proteins with maltose binding protein has been altered. We will determine the sites of these mutations by DNA sequencing and comparing the mutant sequence with the known wild-type sequence. Our long term goal is to generate a picture of how substrate molecules interact with the proteins during the process of translocation through the membrane and how this process is coupled to energy.